Pavements are used to facilitate the passage of wheeled or pedestrian traffic along or over roads, footpaths (sidewalks), playgrounds, and areas used for storage or parking. To do its job well, such a pavement should be relatively smooth and flat. For reasons of economy, such pavements are often cast in substantial lengths, with construction joints between them. However, in some forms, pavements may be formed from preformed slabs made from a settable material, such as concrete, or formed from other rigid material such as steel or wood. Footpaths are pavements that carry relatively light, low speed traffic such as pedestrians and pedestrian vehicles such as wheelchairs, strollers and bicycles. Other categories of light duty pavement include cycle ways, domestic driveways, playgrounds and the like. These pavements generally do not need to be as smooth or flat as those used to carry heavy or high speed traffic.
A pavement is subject to both direct and indirect actions. Direct actions include traffic loads and forces deriving from soil or foundation movement, and tree roots. In the case of footpaths, cycle ways and domestic driveways for example, which are frequently built alongside trees, uplifting actions caused by tree roots are common. Uplifting or depressing actions can be seen as out-of-plane, relative to that of the pavement.
Indirect actions include drying (moisture) and temperature change. When a pavement is made from concrete, these actions cause both temporary and permanent volumetric changes that manifest in the form of expansion and contraction. Shrinkage, which is caused by drying, can be seen in this sense as a form of permanent contraction. The effect of these actions is most significant in the plane of the pavement. For example, the unrestrained drying shrinkage of concrete is commonly in the order of 800 micro strain or 1.2 mm for a slab 1500 mm long. The coefficient of thermal expansion of concrete is commonly in the order of 12 micro strain per degree Celsius or approximately 0.4 mm in a slab 1500 mm long subjected to a temperature change of 20 deg C. If contraction is restrained, it may lead to cracking of the concrete. If expansion is restrained it may lead to any or all of spalling and crushing of the concrete and buckling and warping of the pavement.
Commonly, provision for contraction of concrete pavements is made by incorporating contraction joints at relatively close intervals effectively dividing the pavement into a series of contiguous slabs. In the case of an un-reinforced concrete pavement such as a footpath, for example, contraction joints are commonly spaced at between 15 and 20 times the thickness of the pavement. For a 75 mm thick pavement, this implies joints at 1000 to 1500 mm. Provision for the expansion of concrete pavements, which are subjected to solar heating, such as roads and footpaths, is made by incorporating expansion joints, also known as isolation joints, at relatively wide intervals, commonly 4 to 5 metres. Thus external pavements commonly take the form of a series of contiguous slabs, both separated and linked by a combination of contraction and expansion joints.
For reasons of economy, contraction joints are commonly formed by creating a plane of weakness in the top surface of the concrete, by trowelling grooves in the fresh concrete or cutting grooves in the partially or fully hardened concrete. This encourages cracking to occur at such grooves rather than in a random fashion, which would be unsightly, and helps to create many narrow cracks rather than few large cracks, which would be detrimental. In practice, the effectiveness of this method is subject to variations in the concrete, in the friction between the pavement and the soil or subgrade upon which it rests, workmanship, climatic conditions and other factors, and contraction often accumulates over two or more slabs so that cracks do not occur at some planes of weakness and relatively wide cracks occur at others.
Localised direct actions such as uplifting caused by tree roots or soil heave cause flexural stresses in the pavement. In the case of un-reinforced concrete footpaths for example, which have relatively closely spaced contraction joints, the uplifting action of a tree root will typically lead to the opening or creation of a crack emanating from the top surface of the footpath at a contraction joint adjacent to the point of uplifting. However, the cracking of this construction joint only reduces the flexural strength of a slab significantly in one direction and the aforementioned lifting may lead to the sudden, uncontrolled fracture of the footpath at distances from the point of lifting corresponding to the flexural strength of the concrete. Further, if a crack is relatively wide, a lifted slab may not engage its neighbour with the result that a vertical discontinuity or step will be created in the pavement. In the case of footpaths this often leads to steps of sufficient height to impair the passage of pedestrian vehicles and to cause pedestrians to trip or fall.
Expansion joints usually consist of a sheet of compressible material extending the full thickness of a pavement so as to allow the pavement to expand without inducing excessive compressive stresses in the concrete from which the pavement is made, which could lead to crushing or spalling of the concrete or warping or buckling of the pavement. Such joints have no ability to transfer load or to limit differential displacement within a pavement.